MEMS Clock Cuts Wearable Power in Half

PORTLAND, Ore. — Let's face it: Most of the time our mobile devices are idle. No matter how addicted you've become to checking for messages, surfing the web, listening to music, or playing games, most of the time it's waiting for you to activate it. However, during this idle time your unit is not totally off. If Bluetooth is turned on, for instance, it has to wake up every few seconds to make sure no paired device is wishing to transfer data. Likewise, it has to periodically check for button clicks, WiFi activity, and for battery supervision functions.

During those operations SiTime Corp. of Sunnyvale, Calif., claims its new, high-precision, real-time clock can save 50 percent or more of the power usually expended during idle time, at least doubling the battery life during those periods.

"These power savings might not amount to much for a device with a big battery, but for Internet of Things and wearable devices with tiny batteries, their lifetime can be significantly extended with our new SiT1552 MEMS 32 kHz TCXO [Temperature Compensated Oscillator] real-time clock," Piyush Sevalia, executive vice president of marketing, told EE Times.

Every mobile device has a 32 kHz real-time clock to keep track of time, telling its device when to wake up and perform its duties. Most of these real-time clocks have a precision of 100 to 250 parts per million (PPM), which is adequate to keep a device from seeming unresponsive or sluggish. However, for real-time devices like Bluetooth accessories, even 20 PPM means that the software designers have to include buffer time to make sure the external device is connected properly before data is transferred.

In essence, this means the software designers add extra time at the beginning of a wakeup operation to make sure both devices are communicating, then more extra time at the end of an operation before the device is put back in idle mode to make sure all the data was transferred. However, with SiTimes SiT1552 TCXO real-time clock, its accuracy is within 5 PPM -- the lowest in the industry -- permitting the software writers to drastically cut down those buffer periods, thus saving power and extending battery life, especially for wearables.

"We weren't aware of this as a problem when we started talking to our customers about a TCXO version of our real-time clock, but they immediately told us about how a high-precision, real-time clock could significantly extend battery life -- especially on wearables," Sevalia told us.

The SiT1552 TCXO uses the same TempFlat technology used in its SiT15xx -- a 20 to 100 PPM part announced last year. By adding on-chip temperature sensor and an analog temperature-to-digital converter (ADC) SiTime was able to increase precision to plus or minus 5 PPM. The SiT1552 TCXO real-time clock also claims the smallest footprint of any TCXO today, a 1.5 x 0.8 x 0.55 millimeter chip scale package (CSP), one sixth the size of quartz crystals. And it consumes less than one microAmp of power. The MEMS chip is so small and thin -- 400 x 400 x 125 microns -- that it fits between the solder balls on the bottom of the application-specific integrated circuit (ASIC) containing the electronics.

The SiT1552 is also marketed to device makers that need accuracy over long periods, such as smart meters, which need to stay accurate for 20 years or more. The device has a lifetime of 500 million hours and is covered by SiTime's lifetime replacement warranty, as are all its timing chips. The SiT1552 is in mass production now, and so far SiTime has shipped 220 million total timing chips since its founding in 2005.

At this point, the SiT1552 32 kHz MEMS TCXO is not offered as a VCXO (VCMO).

Typically, we've seen the use of MHz frequencies in discrete PLLs, and so we do offer MHz oscillators and TCXOs with the voltage control (VC) function. Pullability is up to 1600 PPM on the oscillators. Datasheets for those devices are here.

http://www.sitime.com/products/voltage-controlled-vcxo

We also offer the ability to serially pull the oscillators using a digital bit-stream over a 1-wire interface. More information on these DCXOs is available here.

Thank you for your comments. Yes, the SiT1552 is a highly-stable, 5PPM MEMS TCXO (oscillator). It is not equivalent to the DS3231 Real Time Clock IC, though Maxim does offer a DS3231M which is based on a MEMS reference resonator.

On the pricing - the SiT1552 is available for $1.90 in low-volume quantities (10KU EAU). However, like any other semiconductor, pricing drops signficantly in high volumes because of the benefit of scale. If you would like higher volume pricing, please feel free to email me with project details or contact a SiTime representative, and we will be happy to help you.

Reading the SiT1552 data sheet, I realize it is only an oscillator. The term "real-time clock", or RTC, is usually only used when the device contains additional dividers and registers to provide values for hours, minutes, seconds, etc.

Other real-time clock devices with similar accuracy and low power have already been available for quite a few years, such as the Maxim DS3231, which has an accuracy within 3.5ppm over the entire -40 C to +85 C temperature range. But it comes in a package that is much, much larger than the SiT1552.Also, if I understand the data sheets correctly, the DS3231 adjusts its temperature compensation only once every 64 seconds, whereas the SiT1552 does it every 350 msec., which would be much better for rapidly changing temperatures.

Saves 200% of the power? If it used half the power, it would save only 50%, if it saved 100% of the power it would be using no power at all, and if it saved 200% of the power it would be generating power instead of consuming it!!

Regarding the accuracy comment - please see below for a stability plot over temperature of SiTime's 32 kHz MEMS TCXO and a typical 32 kHz tuning fork quartz resonator or oscillator. The SiTime TCXO is much more accuracte (stable) than the quartz device. The two red lines show the envelopes of stability variation of different devices from various manufacturers.

The space gain is impressive, and timing wise it would mean +/- 13 seconds a month or so. This equals +/- 2 minutes and 38 seconds in a year, which means it is as accurate as NON TCXO quartz, unless I'm mistaken. What about power consumption?